Organic Matters
The carbon hidden away in soil is the unglamorous and often forgotten part of the
carbon cycle yet soil organic matter is the key to healthy soils and agricultural
productivity and also represents a natural way to sequester the CO2 that causes global
warming, so researchers are seeking ways to counter its remorseless decline in
Australian farmlands. It’s an important quest because, without organic matter, soil is
essentially sterile.
Soil organic matter comprises all the living, dead and decomposing plants, animals
and microbes in the soil along with the organic residues and humic substances they
release.
‘It includes carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur and it is a
small but vital part of all soils,’ says Dr Jan Skjemstad, formerly of CSIRO Land and
Water. ‘We now recognise four different types – crop residues, particulate organic
matter, humus (usually the largest pool) and recalcitrant organic matter like charcoal –
so it is a complex mixture of materials that vary in size, chemistry, degree of
decomposition and interaction with soil minerals.’
Dr Ram Dalal, of the CRC for Greenhouse Accounting and the Queensland
Department of Natural Resources has also studied soil organic matter in croplands for
many years and has reviewed its role1. He says soil organic matter contains both
living and non-living organic components.
‘Soil microbes, especially fungi and bacteria, and larger soil fauna are the living
component while plant residues and various other organic materials represent the nonliving portion,’ says Dalal ‘and it is the microbes that are the main driving agent for
organic matter turnover.’
The amount of organic matter in a soil results from the balance of carbon inputs
(vegetation and roots) and outputs (decomposition and leaching), offering the
prospect of fiddling with the rates of these to manage or manipulate soil organic
matter.
Sounds promising, but the biological, physical and chemical processes involved are
complex…so boosting organic matter by, say altering cropping systems isn’t always
that simple. For example, there are thresholds of about 3.4 to 7.7 percent (OK??
Evelyn/Jan) above which the various benefits of more soil organic matters levels off
and clay soils respond differently to more sandy soils.
Soil organic matter is universally regarded as good stuff. In a GRDC report2, Dr
Evelyn Krull, of CSIRO Land and Water, and her colleagues, said that many farmers
1
Dalal RC and Chan KY (2001) Soil organic matter in rainfed cropping systems of the Australian
cereal belt. Australian Journal of Soil Research 39:435-464.
2
Krull ES, Skjemstad JO and Baldock JA (2004) Functions of soil organic matter and the effect on soil
properties, GRDC, www.grdc.com.au/growers/res_summ/cso00029/summary.htm
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see increases in soil organic matter as desirable because this is directly related to
better plant nutrition, ease of cultivation and seedbed preparation, greater aggregate
stability, reduced bulk density, improved water holding capacity, enhanced porosity
and earlier warming in spring. At a fundamental level, soil organic matter provides a
reservoir of metabolic energy that enables a range of biological processes to occur in
the soil.
A disappearing act
Many studies have documented the seemingly inevitable decline of soil organic
matter following land clearing and use of conventional cropping systems. To give an
idea of this, researchers3 found that percentage losses of organic carbon from
Australian soils, already low in carbon compared to say European soils, varied from
10-60 percent over 10-80 years of cultivation and other studies have recorded similar
losses. Incidentally, to estimate organic matter, scientists usually multiply the (more
easily determined) soil organic carbon value by a conversion factor of 1.72.
Why are land clearing and cropping so detrimental to soil organic matter?
‘The declining organic matter levels in croplands,’ says Dalal, ‘are mostly due to
changes in temperature, moisture, soil aeration, exposure of new soil surfaces as
aggregates are disrupted, reduced input of organic materials relative to grassland or
forest returns, and increased erosion. An additional problem, easily overlooked, is the
unavoidable export of carbon (?) and nutrients – such as nitrogen, phosphorus and
potassium – that leave the farm each year in grains and other produce.’ (OK Ram?)
These end up on dinner plates all over the world at the expense of rural soils.
Large losses of soil carbon can lead to land degradation while restoration of soil
organic matter improves soil quality and enhances sustainability. Indeed a study4 by
Dalal, and several colleagues, in the Brigalow region of the Murray-Darling Basin
found that soil organic matter is a useful indicator of sustainability and they
confirmed that it declined exponentially as the cultivation period for cereal cropping
increased. As a consequence, soil nitrogen was in shorter supply and grain protein and
grain yields fell, with clear economic implications.
The obvious way to reverse declining organic matter is to put bucket loads of organic
material back into the system and indeed there has been much interest in amending
soils with sewage sludge or other biosolids. Recently, an Australian company has
developed a compost-like medium from recycled urban waste and is keen to see it
returned to where it came from… our farms (see the box).
3
Russell JS and Williams CH (1982) Biogeochemical interactions of carbon, nitrogen, sulphur and
phosphorus in Australian agro-ecosystems. In ‘The cycling of carbon, nitrogen, sulphur and
phosphorus in terrestrial and aquatic systems’. (Eds LE Galbally, JR Freney) pp 61-75 (Australian
Academy of Science:Canberra).
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Dalal RC, Eberhard R, Grantham T and Mayer DG (2003) Application of sustainability indicators,
soil organic matter and electrical conductivity, to resource management in the northern grains region.
Australian Journal of Experimental Agriculture 43(3):253-259.
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Managing soil carbon
We can’t detail here the various land and crop management options and systems for
boosting soil carbon levels. Suffice to say the following aspects and practices feature
prominently:
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use of pasture leys
crop rotations, legumes and fertiliser applications
no till systems and crop residue retention (conservation farming)
no burning of stubble
application of manures or other organics
lower stocking rates on grazing lands
retention of grasslands and trees or conversion of marginal lands to native
vegetation
soil erosion control
Pastures are an important tool because organic matter concentrations tend to be much
higher under grass. Observations5 of the world’s ecosystems show that organic carbon
concentrations in soils (to a depth of one metre) under various land uses were: 122.7
tonnes per hectare for tropical forests, 117.3 for tropical savannas, 96.2 for temperate
forests, just 80 for croplands, and 236 for temperate grasslands.
Similarly, the importance of trees emerged in a recent study conducted by Dr Rick
Young, of the NSW Department of Primary Industries, and several colleagues6.
‘We found that continuous cultivation and cropping over 20 years or more
significantly depleted carbon concentrations compared with grassy woodlands to a
depth of 0.2 metres at all sites and to 0.6 metres at three sites in north-western NSW,’
says Young. ‘The outstanding finding was that comparisons between land uses and
the total amount of carbon stored at the seven sites we studied were dominated by the
number of trees per hectare and the size of the trees.’
The researchers concluded that in most cases, maximum carbon – both soil carbon
and total carbon on site (including biomass carbon) – will be maintained where some
tree cover is retained.
Dr Brian Tunstall of the Environmental Research and Information Consortium,
formerly with CSIRO ……., brings an ecological perspective to the question of
restoring soil organic matter.
‘Unfortunately, more than 75 percent of Australian farming soils have organic carbon
contents less than 1.75 percent,’ says Tunstall, ‘indicating a widespread need to
improve soil organic matter concentrations.’
5
Watson RT, Noble IR, Bolin B, Ravindranath NH, Verardo DJ and Dokken DJ (2000) Land Use,
Land-use Change, and Forestry: A Special Report of the IPCC, Cambridge University Press,
Cambridge , USA.
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Young R, Wilson BR, McLeod M, and Alston C (2005) Carbon storage in the soils and vegetation of
contrasting land uses in northern New South Wales, Australia. Australian Journal of Soil Research,
43:21-31.
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However, he is critical of the current broad-scale farming approach with its high
inputs of mineral fertilisers, herbicides and insecticides, which he regards as
symptomatic treatments that are unsustainable in the long term.
‘We need to increase levels of soil nutrition to increase the amount of organic matter
being recycled,’ argues Tunstall. ‘But, to be sustainable, the nutrition should be
increased by developing the natural processes, a functional soil, rather than treating
soils as a hydroponic-like medium that simply acts as a reservoir for water and
applied nutrients.
‘The idea is to build up biological populations in the soil, not least the
microorganisms in organic matter which maintain a supply of essential elements to
plants, rather than relying only on mineral fertiliser additions which negate the
significance of microbes. A good practical example of this is sugar cane farmers in
Queensland who have been using microbes to incorporate organic matter into the soil
after harvesting.’ (OK Brian?)
Soil carbon, sinks and global warming
Increasing concern about greenhouse emissions and climate change is renewing
interest in soil’s capacity to hold carbon. Perhaps surprisingly, there is more carbon
sequestered in the world’s soils (2500 billion tonnes, GT) than in the atmosphere (807
GT) or living organisms (560 GT). And some 1550 GT of this soil carbon, more than
half, is in soil organic matter.
So the historical losses of soil organic matter represent a considerable contribution to
greenhouse emissions. In Australia, more than half the C02-e (carbon dioxide
equivalent) emissions from our agriculture sector are due to loss of organic carbon
from the cereal belt. The good news is that this is a reversible process and there are
bonus benefits.
‘If we can reverse these cereal belt losses by restoring soil organic matter, there is the
potential to lock up about 50 Mt CO2-e per year in the soil profile over a 20-year
period,’ said Dalal. ‘That’s more than twice the current annual CO2 emissions in
Australia.’ (OK Ram, paraphrased for Ecos). Can we express this as the proportion of
current emissions that could be sequestered each year, rather than over 20 years, to
compare like with like?)
Tunstall calculates that an average organic matter increase of 2 percent to a depth of
30 cm, achievable in many clay soils, represents a sequestration of 35 tons of carbon
or 128 tons of CO2 per hectare…and there are many hectares of farm land. (Brian:
tons or tonnes?)
Krull points out that adopting sensible land management practices to boost soil carbon
has benefits that extend beyond the much-discussed greenhouse gas sequestration.
‘Direct benefits to the landowner include increased productivity, sustainability and,
above all, better soil quality,’ said Krull. ‘While growing forests is an important
sequestration measure, it doesn’t have the advantage of enhancing agricultural
productivity that researchers envisage through rebuilding soil organic matter on
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farms. In the short term, over 25 to 30 years, this is considered the most cost-effective
option for sequestering greenhouse gases.’ (OK Evelyn?)
Besides the greenhouse benefits, there is a view that the next dramatic increase in
crop production could come about by a deliberate increase in the biological activity of
soil and by accessing nutrients through microbe driven mechanisms. One way of
doing this is by addition of organic fertilisers on some farms, which could build soil
carbon levels and, as a likely consequence, increase yields.
Steve Davidson
Sending organics home
One reason for the decline of soil organic matter in agricultural soils is the removal–
some would say ‘strip mining’ of nutrients – in crops and other farm produce. Instead
of being recycled into the soil, as they would in a natural system, plant and animal
material (such as grains and meat) are sent to faraway markets and end up being
consumed by people in towns and cities or manufactured into other foods or goods or
end up, eventually, as household kitchen waste in garbage bins or as human waste in
sewerage.
John White, Managing Director of Global Renewables Limited (GRL), is on a
mission to reverse this trend. He has a vested interest, as GRL collects urban waste
and recycles the various components, including organic wastes, using a process
known as UR-3R. However, he is passionate about the need for sustainable
agriculture in this country and believes that his company can play an important part
by collecting organic waste and returning it to farm soils as a composted material that
will boost soil organic matter concentrations and increase productivity.
‘I believe Australian farmers are losing their soil organic matter and, encouraged by
fertiliser companies, are increasingly addicted to synthetic fertilisers and pesticides,’
White told Ecos. ‘And it is a vicious cycle because as soil organic matter declines,
soils degrade, microbial activity declines and farmers must use more and more
chemicals to maintain fertility, control weeds and pests and fight plant diseases. (?OK
John, paraphrased for brevity)
‘This is all detrimental to the environment, in particular as nutrients enter our rivers,
lakes, groundwater, and even marine waters, causing algal blooms, damage to coral
reefs and so on. Also, loss of soil organic matter means soils retain less water, so
farms are more vulnerable to the effects of drought,’ White said.
‘Some 40-60 percent of the waste in garbage bins is organic – food scraps, garden
clippings, grass – and this amounts to 10-18 million tonnes (OK John??) going to
Australian landfill sites each year,’ said White. ‘At GRL, we separate this out, clean
and deodorise it, and use microbes to process it into a compost suitable for
agricultural soils… so we close the loop…sending organic matter back to farms, in
much the same way that small-scale traditional village systems returned wastes to the
soil.’
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White has been urging the Australian government to follow the lead of the United
Kingdom where laws were passed last year to progressively ban organics at landfills
over the next decade to minimise pollution of water tables through leaching and
reduce greenhouse gas emissions. He argues that government incentives are needed to
encourage farmers to manage their soils sustainable and suggests that organic
farming, already booming, is the best way to achieve this.
‘Our government is beginning to get interested,’ said White, ‘perhaps motivated by
greenhouse pollution problems, farmers going bankrupt and the likelihood that
European countries, especially, will soon demand truly clean and green produce in
international trading.’
Some scientists urge caution on soil amendments. Professor Keith Cameron and his
colleagues at Lincoln University, New Zealand, argue that while wastes may contain
nutrients and, used properly, can serve as valuable fertilisers for agricultural
production, they should not just be ‘dumped’ on soils.
‘There needs to be good management of waste composition and application rate and
appropriate guidelines to minimise damage and maximise benefits,’ said Cameron.
‘Recycling nutrients can improve sustainability, but excessive rates of application
have been shown to cause N and P pollution of surface or groundwater and some
wastes contain heavy metals, salts and pathogens or may have extremely low or high
pH.
‘Land application of wastes can have a variety of beneficial or detrimental effects on
soil conditions, depending on the nature of the waste and the soil,’ Cameron said.
‘Research and monitoring programs are therefore necessary to ensure that waste
application systems are sustainable and do not damage soil quality.’
John Lawson, senior engineer ?at GRL, says the U3-3R process, as employed at
plants in New South Wales and in Lancashire, England, removes glass, metals,
plastics and other objects from the organic component and the resulting processed
compost meets the Australian standard. He said in trials it has boosted yields in a
range of irrigated horticultural crops from tomatoes to zucchinis and could make a
significant contribution to organic farming systems. (??OK John L)
www.globalrenewables.com.au
Cameron, KC, Di HJ and McLaren RG (1997) Is soil an appropriate dumping ground
for out wastes? Australian Journal of Soil Research, 35, 995-1035.
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